Currently Mobile Telecommunications Network Operators (MNOs) are working with more and more technologies, using different frequencies and bands. To be able to achieve the demand of these multi technologies, the Operators will need to place different antennas for different technologies, making the site enormous.
Other solution will be to use multi-band antennas which are big in length and width when compared to existing single band antennas, making the site or building unattractive for the everyday viewer and hard to install.
The ideal solution would be a multi-band antenna with small size and good directivity. There is a good candidate for a compact and high-directivity antenna: the Yagi antenna.                The Yagi (or Yagi-Uda) antenna is a linear array of parallel dipoles. One element is energised directly by a feed transmission line with the others acting as parasitic radiators.        The Yagi-Uda is built for one frequency. Reflector and Directors length and also the spacing between them is calculated depending of the Yagi work frequency.        This structure makes the Yagi a good directivity antenna.        
Yagi antenna is a directional antenna system, it has one dipole connected to the transmission line and a number of equally spaced unconnected dipoles mounted parallel to the first in the same horizontal plane to serve as directors and reflectors, it is a but it only works in one band.
There are some proposals in the art which are actually related to this problem, the so called quasi-yagi antennas. Those antennas have one director, affecting the performance of the antenna; and they have either passive or active elements limiting the performance as well.
Current Single RAN (Radio Access Network) products are only Single Band. There is a big challenge to implement the wideband solution with current passive antennas.
Each one of the antenna elements is fed with an Adaptive Antenna Systems (AAS) element, where the antenna element can be the deep antenna solution or the traditional antenna solutions; the main challenge for the current Wideband Single RAN (Radio Access Network) solutions is the duplexer, which allows a transmitter operating on one frequency and a receiver operating on a different frequency to share one common antenna with a minimum of interaction and degradation of the different RF signals.
Radio receivers can be damaged if high level RF signals, like those directly from a transmitter output, is applied to the receiver antenna. Additionally, receivers may become ‘desensitized’ (or ‘de-sensed’) and not receive weak signals when high noise levels or another signal near the receive frequency is present at the receivers antenna input.
Obviously, radio receivers and transmitters cannot be directly connected to the same antenna without some device being used to:                Switch the antenna between the transmitter and receiver so that they are never connected to the same antenna at the same time.        When the transmit and receive frequencies are different, filters may be used to reduce the transmit signal levels to an acceptable low level at the receivers antenna input. Naturally, you cannot filter out the transmitter signal when it is the same as the receiver frequency.        
As stated above, in Single band Adaptive Antenna Systems (AAS) with current antennas there is one Tx (transmission) band and one Rx (reception) band.